MARSIDA LAZE, LINDITA HAMOLLI, MIMOZA HAFIZI
Abstract
Gravitational microlensing phenomenon occurs when compact objects pass in front of background sources, bending and magnifying their light. However, parameters derived from standard photometric microlensing light curves are affected by strong degeneracies, preventing a unique determination of the lens properties. These degeneracies can be partially broken by measuring second order effects, such as parallax and finite-source effects, as well as through astrometric observations. Polarimetric measurements during microlensing events provide an additional and promising approach. In particular, light from stars with extended atmospheres can become polarized through scattering processes within their circumstellar envelopes. Microlensing effect can break spherical symmetry, resulting in a detectable polarization signal, particularly for stars with extended atmosphere. Polarimetric observations can constrain the Einstein angular radius, the direction of the lens trajectory, and the transverse velocity. Considering the future microlensing observations towards the Galactic bulge by the Nancy Grace Roman Space Telescope, we explore possibility of detection of polarization signal by FORS2 at the Very Large Telescope (VLT). We find that the lens with mass in range [10−3, 10] 𝑀⊙ and distances up to 5kpc can produce a measurable polarization signal and their detection probability is 1.6%. We emphasize that these events are important because, beyond the constraints on the lens parameters that can be found by second-order effects, they also enable the determination of the direction of the lens movement, which is especially important when the lenses are dark objects.
Key words: microlensing, polarization, extended envelope.